Over the years, many types of drills and reamers have been developed which have found utilization in various machining operations. For example, so-called "spade drills" have found utility in connection with woodworking applications, and twist drills, characterized by having one or two spiral grooves machined into the shank of the drill/reamer have been used for metal and other harder materials, with the grooves or "flutes" being useful in furnishing lubricant to the surface being machined as well as removing chips from the path of the drilling operation. It has been found that, although considerable development has been accomplished in the fields of high-speed turning and milling, very little research has been devoted to high-speed drilling. Conventional methods of removing metal at high speeds presently used in industry call for tool velocities extending between approximately 35 to 450 surface feet per minute (SFM) for most steels. For aluminum and similar metals, various charts recommend speeds up to 1800 SFM. These slow cutting speeds induce surface and residual stresses into the part being machined or drilled, causing a warped or out-of-flatness condition, especially in thin parts. Excessive numbers of passes are required to obtain surface finishes desired. In drilling at conventional speeds it is virtually impossible to hold tolerances of less than 0.004 inches of the hole diameter without a ream finish operation. Also, when drilling at slower speeds, i.e., in the neighborhood of 8" feed rate per minute, excessive exit burrs are experienced.
It has been an objective to increase the speed at which metal removal can occur so as to increase productivity in manufacturing operations, thus lowering costs.
Illustrative of cost/productivity considerations is a phenomena which occurs in ultra high-speed milling or drilling in which horsepower requirements of a high-speed spindle drop dramatically when optimum cutting speeds or chip removal is attained. For example, under conventional methods of drilling a 5/32 inch diameter hole in 1/2 inch thick aluminum, approximately 5 to 10 horsepower at 38 inches per minute feed rate is required. Utilizing a drill/reamer having a high-speed spindle with a tool geometry similar to that defined in the instant invention, it has been found that this same hole can be drilled at 25,000 rpm at 38 inches per minute feed rate with less than one horsepower! The cost of a 10 horsepower high-speed spindle is several thousands of dollars more than a 1 horsepower spindle, a saving which is significant in manufacturing operations.
One of the criteria for success in high-speed drilling is to efficiently remove chips from the hole. Depending on RPM, optimum spiral flute angles have been found which have been successful in chip removal and in eliminating the "bunching" of chips. With high-speed cutting, the bulk of generated heat is contained in the chips which are removed, and the drill and part being machined stay relatively cool.
It has also been found that hole wall finish improves with higher rates of drilling. Furthermore, exit and entry burrs were minimized with a prototype of the instant drill design used with a proper backup plate. Hole diameters produced with such a drill were equal to the drill diameter plus 0.0005 inch on a one-shot drilling basis. A high-rate of retraction (Z direction) is desirable not only to reduce hole rifling, and trapped chips and fragments, but also to prevent heat buildup which accentuates the problem of warpage mentioned above.